The present invention relates generally to a cable tensioner, in particular to a cable tensioner for a window regulator mechanism.
A window regulator mechanism is a device used in a motor vehicle to transmit a drive force to a window. A window regulator device includes a transmission device (such as a cable or a belt) that is connected to a drive device (such as a crank handle or a motor) which displaces the window. The window can be driven by sliders which move along guide rails under the action of the cable. The cable can include a lower cable and an upper cable wound in opposite directions on a drum that is driven by the motor or the crank handle.
The tension of the cable in the window regulator mechanism must be correctly controlled throughout the duration of its service life. Slack in the cable loop can cause inaccurate displacement of the window with respect to the instruction provided by the drive system.
The operational accuracy of the widow regulator mechanism is important and is particularly important in frameless door window regulator mechanisms. In certain models of frameless door window regulator mechanisms, the window lowers slightly when the door is opened to release itself from a roof seal. The displacement of the window must therefore be carried out accurately to not impede with the opening of the door. However, the displacement must not be too great in order to comply with official restrictions, in particular regulations regarding anti-pinching. The drive cable of the window must therefore have sufficient tension to guarantee an accurate displacement of the window.
The components forming the window regulator mechanism device progressively age, which apparently elongates the cable due, for example, to the wear of the drive drum and of the pulleys, the compression of the cable covers, or the creep of the pulleys. Cable elongation due to the aging of the various parts of the window regulator mechanism must be compensated for.
Mechanisms for taking up play are known that absorb the elongation of the cable to guarantee a tension in the cable that is sufficient for the correct functioning of the window regulator mechanism device. Most of these mechanism are systems that operate in a step by step mode, based on more or less fine notches, such as the one described in the patent DE 197 06 866 A.
To guarantee better regularity of the tension in the cable, it is preferable to employ a continuous take-up system which avoids threshold effects. Such a mechanism is described in the parent application EP-A-0 244 303 and includes a tensioner screw and nut having a force applied to it by a push rod and an elastic member. In one direction, the screw can, under the axial thrust of the elastic member, helically move in the nut each time the force applied by the push rod disappears or reduces. In the other direction, the screw cannot rotate or translate in the nut. This mechanism is an irreversible system based on friction. Therefore, when play is likely to occur between two parts associated with the tensioner screw, play is automatically compensated for when it appears by the unidirectional displacement of the screw.
The cable tensioners are disposed on a given cable path, that is on the lower cable or the upper cable of the window regulator mechanism device. The cable tensioners are generally placed on the lower cable of the window regulator mechanism, which is the “slack section” of the mechanism when the mechanism is at the end of upward travel. To tension the upper cable, it is necessary to provide another cable tensioner. However, this represents a significant cost overhead. The simultaneous tension of the lower cable and the upper cable makes it possible to balance the tension of one with respect to the other and eliminates any risk of the cables disengaging from their slides. The cable tensioners do not allow simultaneous tension of both the lower cable and the upper cable.
Moreover, the cable tensioners are irreversible systems. That is, when the unidirectional displacement screw has compensated for insufficient tension in the cable, it cannot carry out the reverse movement if there is excess tension on the cable. These mechanisms are well suited to compensate for permanent elongation of the cable due to aging of certain parts of the device. In certain cases, the cable can exhibit insufficient tension that is not permanent, but simply a temporary insufficient tension due, for example, to the stop of the window in the top or bottom position. If the cable tensioner compensates for such temporary elongation, the cable can subsequently have excess tension, which risks accelerating premature wear.
U.S. Pat. No. 4,235,046 discloses a cable tensioning mechanism designed to maintain the tension of a cable when the cable is driven in two opposite directions, that is to maintain the tension on the lower cable and on the upper cable. The mechanism includes a leaf spring having a semicircular shape in a rest state, and each end of the leaf spring includes a groove for engaging the cable. The leaf spring is centered by a fixing point on the drum for winding the cable sections. The leaves apply a force on each of the lower section and the upper section of the cable, and the leaf spring causes a local inversion of curvature of the cable. The leaf spring is, however, costly when it is dimensioned to induce a force sufficient to provide tension in the drive cable. In fact, the leaf spring must have rather long and largely dimensioned branches to not damage the cable by curvature inversions that are too great.
There is therefore a need for an inexpensive cable tension mechanism for a window regulator mechanism device which is not completely irreversible and which simultaneously tensions both the lower cable and the upper cable of the device.